We have a 40' DP and have pretty well gotten into every place we've wanted to. Granted, some RV parks / campgrounds have limited spaces for the larger rigs so we do have to make some reservations. You should see some of the places we've had it off boondocking.
A lot of people buy three MHs before they're done and should have bought the last (larger) one first and they would have saved money.
So if you're willing to do a little planning, you should be alright with a 40' rig. If you don't do any planning and just do things spur of the moment, stay with the smaller rig to be able to get into anyplace.
And with larger rig, you need to be good at backing up to get it into some places.
Bill